but also any change in the protocol would need the acceptance of the majority of bitcoin users.
Even if SHA-256 is partially broken (say easier to find hashes, but not fully broken), you-d see major resistance from ASIC miners. You might even get a split network.
Whoever holds the crowd holds the coinvalue.
We'd be on the 3rd or 4th generation of ASICs by the time SHA-256 needs to be replaced under any realistic conditions, but even then the transitional process wouldn't likely make the ASIC's completely worthless. For example, the crypto setup for the blockchain (wherein SHA-256 is used) has "hooks" for using two different algo's in series. Currently, SHA-256 is simply used twice, but if things start looking like SHA-256 is at risk of being undermined; the developers could switch one of those methods to some other algo, so that both SHA-256 and the new algo must be employed. This would still give those professional miners with SHA-256 ASICs an advantage over GPUs for a time, as well as a future set of algos in order to develop the next set of ASICs for. If SHA-256 is ever broken, it won't matter much what the ASIC miners had planned.